1. Field of the Invention
The present invention relates to a power supply apparatus having a transformer tap-connected to a DC power supply, which is connected at both ends of the primary side to a plurality of switching means for converting a DC voltage into a AC voltage, and output an AC voltage induced to the secondary side.
2. Description of the Related Art
FIG. 1 is a block diagram showing the structure of a conventional power supply apparatus for a traveling wave tube. As shown in FIG. 1, the conventional power supply apparatus comprises DC power supply 1, input filter 2, voltage stabilizing circuit 3, and step-up transformer 8. The DC power supply 1 outputs a DC voltage of Ei. The input filter 2 eliminates noise from the DC voltage supplied from the DC power supply 1. The voltage stabilizing circuit 3 primarily stabilizes the voltage received from the input filter 2. The step-up transformer 8 receives the DC voltage that is output from the voltage stabilizing circuit 3 at a center tap of the primary side thereof.
Switching devices 6 and 7 are respectively connected at each end of the primary side of the step-up transformer 8. The switching devices 6 and 7 turn on and off an output voltage of the step-up transformer 8 so as to generate an AC voltage from a DC voltage. A rectifying circuit 9 and a traveling wave tube 10 are disposed at the secondary side of the step-up transformer 8. The rectifying circuit 9 rectifies an AC voltage induced on the secondary side of the step-up transformer 8.
The voltage stabilizing circuit 3 uses a boost regulator or a back regulator. The boost regulator compensates the voltage fluctuation of the DC power supply 1 and raises the DC voltage with a predetermined ratio of transformation. The back regulator compensates the voltage fluctuation of the DC power supply 1 and lowers the DC voltage with a predetermined ratio of transformation. Each of the switching devices 6 and 7 is composed of a transistor such as a MOSFET, a bipolar transistor and driven by an external driving circuit (not shown).
In the step-up transformer 8, a first part of a primary side coil, a primary side coil, and a second part of the primary side coil are successively wound around a core. Thus, the degree of coupling between a primary side and a primary side is improved and thereby the leakage inductance is reduced. The rectifying circuit 9 may be a full-wave rectifying circuit or a half-wave rectifying circuit.
FIGS. 2A, 2B, 2C, 2D, 2E, and 2F are time charts showing waveforms of signals of individual portions of the conventional power supply apparatus for the traveling wave tube. With reference to FIGS. 2A to 2F, the operation of the conventional power supply apparatus for the traveling wave tube. In FIGS. 2A to 2F, vertical lines represent on/off switching timings of the switching devices 6 and 7. The output voltage signal of the DC power supply 1 is supplied to the input filter 2. The input filter 2 eliminates noise from the voltage signal. The noise-free voltage signal is supplied to the stabilizing circuit 3.
The stabilizing circuit 3 primarily stabilizes the input DC voltage and outputs a stabilized voltage. As shown in FIG. 2A, the output DC voltage Vi lowers between timing of turning on and timing of turning off of the switching devices 6 and 7. This is because a current I shown in FIG. 2B flows through the step-up transformer 8. The current I varies corresponding to the power consumption of the traveling wave tube 10.
The current I is supplied to a center tap of the step-up transformer 8. When the switching device 6 is turned on and the switching device 7 is turned off, a current id1 as shown in FIG. 2C flows through the switching device 6 on the primary side of the step-up transformer 8. Whenever the switching devices 6 and 7 are turned on or off, the level of a voltage Vds1 applied between the terminals of the switching device 6 becomes High or Low, respectively. The maximum values of the voltage Vds1 is 2Vi as shown in FIG. 2D.
On the other hand, when the switching device 6 is turned off and the switching device 7 is turned on, a current id2 as shown in FIG. 2E flows through the switching device 7. Whenever the switching devices 6 and 7 are turned on and off, the level of a voltage Vds2 applied between the terminals of the switching device 7 becomes L and H, respectively, as shown in FIG. 2D. The maximum value of the voltage Vds2 is 2Vi as shown in FIG. 2F.
The currents id1 and id2 that flow through the switching devices 6 and 7 return to the DC power supply 1 through the stabilizing circuit 3 and the input filter 2. A voltage that is induced on the secondary side of the step-up transformer 8 is input to the rectifying circuit 9. The switching operations of the switching devices 6 and 7 generate an AC voltage. For that reason, an output AC voltage on the secondary side of the step-up transformer 8 is supplied to the rectifying circuit 9. The rectifying circuit 9 rectifies the induced voltage. The rectified voltage is applied to the traveling wave tube 10. With the rectified voltage, the traveling wave tube 10 is driven.
In recent years, the requirement of the size reduction of the power supply apparatus is becoming strong. In particular, the size reduction of the step-up transformer that occupies a relatively large space of the power supply apparatus is being required. However, to reduce the size of the step-up transformer, it is necessary to decrease the winding volume of coils and so forth. Thus, the step-up transformer cannot be fabricated in the conventional method. In addition, when the size of the booting transformer is reduced, the leakage inductance may increase. When the leakage inductance increases, the input current I of the step-up transformer may fluctuate.
FIGS. 3A, 3B, and 3C are time charts showing current values of currents id1 and id2 supplied to the switching devices 6 and 7 in the case that the input current I fluctuates. FIG. 3A shows the input current I. FIGS. 3B and 3C show the current values of the current id1 and id2, respectively. As shown in FIG. 3A, when the input current I fluctuates, the current values of the currents id1 and id2 become different. This situation causes switching noise to increase and the switching devices to damage. In addition, when the current values of the currents id1 and id2 are different, the copper loss takes place in the step-up transformer. Thus, the efficiency of the power supply apparatus may deteriorate.